1. Field of the Invention
The present invention relates to a field emission cathode, an electron emission device and an electron emission device manufacturing method.
2. Description of the Related Art
Various types of flat display devices each having a field emission cathode, i.e., panel display devices, have been proposed. To realize bright picture display, a cathode-ray tube configuration for impacting an electron beam on a fluorescent screen serving as a picture formation surface to thereby emit light is normally adopted.
In a conventional flat display device having a cathode-ray tube configuration, as proposed in Japanese laid-open patent publication No. 1-173555, for example, a plurality of thermionic emission cathodes, i.e., filaments are provided to face a fluorescent screen, thermions generated from the cathodes and secondary electrons resultant from the thermions are directed toward the fluorescent screen to thereby excite and emit the fluorescent screen having colors according to a video signal using an electron beam. In this case, as a display screen becomes larger in size, a constitution in which common filaments are provided for many pixels, i.e., many red, green and blue fluorescent trios forming a fluorescent screen, is adopted.
Therefore, as the display screen becomes larger in size,it becomes complicated to arrange and assemble the filaments. Further, with a view of making a flat display device of cathode-ray tube configuration smaller in size, the depth of the device is made shorter by shortening an electron gun and increasing the deviation angle of electrons. However, since the display screen of a flat display device is increasingly made larger in size, the development of thinner, flat display devices is further desired.
In light of the above respects, a flat display device employing field emission cathodes or so-called cold cathodes has been proposed for the conventional flat display device. In case of the electron emission device having cold cathodes, the selection of a cathode material and a method of forming the cold cathodes constitute important factors in the determination of device performance. The conventional field emission cathode employs high melting point metal such as Mo, Ni and W, or Si for the material of an emitter which emits electrons.
Further, there is proposed a so-called Spindt type electron emission section constituting a flat display device having a conventional structure.
The structure of one example of a conventional flat display device 100 will be described with reference to the drawing.
FIG. 14 is a schematic perspective view of a flat display device 100 having a conventional structure.
The flat display device 100 has a fluorescent screen 101, a flat white light emission display device main body 102 having field emission cathodes K arranged to face the fluorescent screen 101, and a flat color shutter 103 arranged to contact with or face the front surface at which the fluorescent screen 101 is arranged.
As shown in FIG. 14, the display device main body 102 has a light transmission front panel 104 and a back panel 105 facing each other through a spacer (not shown) for holding the panels 104 and 105 at a predetermined distance therebetween, the peripheral portions thereof are airtight sealed by a glass frit or the like, and a flat space is formed between the panels 104 and 105.
An anode metal layer 160 and a fluorescent screen 101 having a white light emission fluorescent materral bonded on the entire surface are formed on the inner surface of the front panel 104. A metallized layer 106 such as an Al film is bonded on the resultant surface as in the case of an ordinary cathode-ray tube.
On the other hand, many cathode electrodes 107 extending perpendicularly in, for example, a band manner are arranged in parallel and bonded on the inner surface of the back panel 105.
An insulating film 108 is bonded on the cathode electrodes 107 and gate electrodes 109 extending in a direction almost orthogonal to the extension direction of the cathode electrodes 107, e.g., in a horizontal direction, are arranged in parallel.
Opening holes 110 are perforated at crossings at which the cathode electrodes 107 and the gate electrodes 109 cross one another. Conical field emission cathodes K are bonded and formed on the cathode electrodes 107 in each opening hole 110.
The field emission cathodes K are formed by using high melting point metal such as Mo, W or Cr, or Si. The cathodes K are of conical shape with a tip end thereof having a radius of curvature of several tens of nanometers and directed toward the gate electrode side.
If a positive voltage of several tens of volts is applied to the gate electrodes relative to the cathode electrodes, a electric field of, for example, about 106 to 107 V/cm is applied to the conical tip end portions and electrons are emitted therefrom by a tunnel effect.
The emitted electrons are allowed to impact on the fluorescent screen 101 formed on the anode electrodes facing the cathodes K at a distance of 0.2 mm to 1 mm therebetween, thereby obtaining fluorescence emission.
One pixel of the flat display device 100 consists of several tens to several thousands of Spindt-type electron emission sections. To structure a display having 1024xc3x97768xc3x97(RGB) pixels of XGA class which is the standard class of a computer display, for example, 100 million to 100 billion electron emission sections are required.
The constitution of a cathode structure including the field emission cathodes K, the gate electrodes and the like constituting the flat display device 100 having the conventional structure will be described with reference to the manufacturing step views shown in FIGS. 15 to 18, together with one example of a manufacturing method to facilitate understanding the cathode structure.
First, as already described above with reference to FIG. 14, cathode electrodes 107 are formed on the inner surface of the back panel 105 in one direction, e.g., in a perpendicular scan direction.
Each cathode electrode 107 is formed into a predetermined pattern by, for example, forming a metal layer such as a Cr layer on an entire surface by deposition, sputtering or the like and then selectively etching the metal layer by photolithography.
Next, as shown in FIG. 15, an insulating layer 108 is bonded on the entire surfaces of the cathode electrodes 107 thus patterned by sputtering or the like. Further, metal 111 such as high melting point metal of Mo or W, finally constituting gate electrodes 109, is formed on the insulating layer 108 by deposition, sputtering or the like.
Next, as shown in FIG. 16, a resist pattern (not shown) made by a photoresist or the like is formed and the metal film 111 is subjected to anisotropic etching, e.g., RIE (reactive ion etching) using the resist pattern as a mask, thereby forming band-shaped cathode electrodes 109 into a predetermined pattern, i.e., in a horizontal direction orthogonal to the extension direction of the cathode electrodes 107 shown in FIG. 14. In addition, a plurality of small holes 111h, for example, are formed in portions where the gate electrodes 109 cross the cathode electrodes 107.
Next, through these holes 111h, etching, e.g., chemical etching by which the gate electrodes 109, i.e., the metal layer 111 is not etched and the insulating layer 108 is isotropically etched, is performed to thereby form opening holes 112 each having a larger width than the width of a small hole 111h and having a depth corresponding to the entire thickness of the insulating layer 108.
In this way, as shown in FIG. 14, the opening holes 110 each consisting of the opening hole 112 and the small hole 111h are formed at crossings at which the cathode electrodes 107 and the gate electrodes 109 cross one another.
Next, as shown in FIG. 17, a metal layer 113 made of, for example, Al, Ni or the like is bonded on the gate electrodes 109 by oblique deposition.
The oblique deposition is carried out while rotating the back panel 105 within the plane thereof and round holes 114 each having a conical inner periphery are formed on surroundings above the small holes 111h. 
In this case, the metal layer 113 is deposited while setting an angle so that the inside of the opening holes 112 is not deposited into through the small holes 111h. 
Thereafter, a field emission cathode material, i.e., a metal having a high melting point and a low work function such as W or Mo, is bonded on the cathode electrodes 107 within the opening portions 112 perpendicularly to the cathode electrode surfaces through the round holes 114 by deposition, sputtering or the like. In this case, even if the deposition is carried out perpendicularly, the cathode material is formed to have an oblique surface continuous to the oblique surface of the metal layer 113 on the surroundings above the round holes 114. Thus, if the thickness of the deposited material reaches a certain level, the round holes 114 get closed. Due to this, dot-like, conical cathodes K each having a triangular cross section are formed in the respective opening holes 112 on the cathode electrodes 107.
Then, as shown in FIG. 18, the metal layer 113 and the cathode material formed on the metal material 113 are removed, thereby forming conical, dot-like cathodes each having a triangular cross section in the respective opening holes 110 on the band-shaped, i.e., stripe-shaped cathode electrodes 107.
The insulating layer 108 exists around the cathodes K, whereby the cathodes K are electrically isolated from the cathode electrodes 107 and a cathode structure having gate electrodes 109 in which electron beam transmission holes are formed by the above-stated small holes 111h and arranged to face the respective cathodes K, is formed.
In this way, the cathode structure in which the field emission cathodes K are formed on the cathode electrodes 107 and the gate electrodes 109 are formed across the upper portions of the cathodes K, is arranged to face the white fluorescent screen 101.
In the display device main body 102 constituted as stated above, a positive, high anode voltage relative to the cathodes is applied to the fluorescent screen 101, i.e., a metallized layer 106, and a voltage sufficient to allow electrons to be sequentially emitted between, for example, the cathode electrodes 107 and the gate electrodes 109 from, for example, the field emission cathodes provided at the crossings where the cathode electrodes 107 and the gate electrodes 109 cross one another, e.g., a voltage of 100V is applied to the gate electrodes 109 relative to the cathode electrodes 107 while modifying sequentially and according to display content, thereby directing electron beams from the tip end portions of the cathodes K toward the white fluorescent screen 101.
Thus, the display device main body 102 makes it possible to obtain white pictures in light emission patterns corresponding to the respective colors in a time division manner and to switch over the color shutter 103 synchronously with the time-division display to thereby fetch light corresponding to the respective colors. In other words, red, green and blue optical images are sequentially fetched, whereby color picture display is carried out as a whole.
As already stated above, in the flat display device 100 of the conventional structure shown in FIG. 14, the field emission cathodes K which face the fluorescent screen are each formed into a conical shape having a triangular cross section in the manufacturing steps described with reference to FIGS. 15 to 18 and an electric field is concentrated on the tip end portions of the conical cathodes K to thereby emit electrons.
However, due to the current development of technology, there is demand for forming the electron emission sections of the field emission cathodes K constituting the flat display device 100 of this type more economically.
Moreover, as already described above with reference to FIGS. 15 to 18, it is known that if the field emission cathodes K are formed out of a material, such as Mo or W, having a work function of 4 to 5 eV, a higher voltage needs to be applied so as to obtain necessary emission current density.
Meanwhile, it is necessary to efficiently concentrate an electric field and to efficiently emit electrons by making the electron emission sections sharper or forming the electron emission sections out of a material having a smaller work function so as to meet the recent demand of low power consumption.
To solve the above disadvantages, there is proposed, in Japanese laid-open patent publication No. 10-357928, a technique for employing conductive, plate-like fine particles for electron emission sections.
Further, as examples of using a material having a small work function for a cold cathode, Japanese laid-open patent publication Nos. 50-81060, 54-51776 and 6-36688 disclose techniques employing alkali metal and alkaline-earth metal nitrides.
The techniques proposed in the above publications are, however, applied to the cold cathode of a gas discharge tube and no consideration has been conventionally given to the application of such techniques to a field emission cathode.
The present invention has been made after the inventors of the present invention were long devoted to studies to solve the above disadvantages. It is, therefore, an object of the present invention to provide a field emission cathode, an electron emission device and an electron emission device manufacturing method capable of realizing efficient field emission by making the electron emission sections of a field emission cathode K constituting a flat display device smaller in size, making the tip end portions of the sections sharper, particularly limiting a work function to 2 to 3 eV to thereby bonding a material having a small work function on the surface of the field emission cathode K.
A field emission cathode according to the present invention is arranged to face an electron applied surface, wherein at least an electron emission section of the field emission cathode is formed out of conductive, thin-plate like fine particles; and a substance having a work function of 2 to 3 eV is bonded on surfaces of the conductive, thin plate-like fine particles.
An electron emission device according to the present invention is an electron emission device having a field emission cathode arranged to face a fluorescent screen, wherein the field emission cathode K constituting the electron emission device according to the present invention is constituted such that at least an electron emission section is formed out of conductive, thin plate-like fine particles; the field emission cathode K is constituted in a state in which a substance having a work function of 2 to 3 eV is bonded on surfaces of the conductive, thin plate-like fine particles; by applying an electric field electrons are emitted from an end face of the electron emission section consisting of the thin plate-like fine particles, of the electron emission cathode.
An electron emission device manufacturing method according to the present invention comprises the steps of: forming a photoresist pattern having small holes on a surface on which a field emission cathode constituting the electron emission device is formed, each of the small holes arranged regularly in advance and having a depth reaching the surface on which the field emission cathode is formed; preparing a coating agent from the conductive, thin plate-like fine particles, at least one of alkaline-earth metal, alkali metal, an alkaline-earth metal compound and an alkali metal compound, a dispersing agent and a solvent; coating the coating agent on the photoresist pattern and drying the photoresist pattern coated with the coating agent; removing the photoresist pattern; and conducting baking, evacuation and sealing operations at a temperature at which the alkaline-earth metal compound or the alkali metal compound is decomposed; and forming the electron emission cathode, in a state in which a substance having a work function of 2 to 3 eV is bonded on surfaces of the conductive, thin plate-like fine particles.
According to the field emission cathode of the present invention and the electron emission device including the field emission cathode of the present invention as a constituent element, the electron emission section of the field emission cathode K is formed out of thin plate-like fine particles. Due to this, if an electric field is applied to the electron emission section, the electron beam emission section is made sharper.
Further, the field emission cathode K is constituted such that an electron emission substance having a work function of 2 to 3 eV is bonded on the surfaces of the conductive, thin plate-like fine particles. Since the substance having a work function of 2 to 3 eV is bonded on the surfaces of the thin plate-like fine particles constituting the field emission cathode while the carbon constituting the field emission cathode has work function of about 4.7 eV, it is possible to particularly decrease the apparent work function of the electron emission section of the field emission cathode relative to the work function of carbon. Thus, the threshold voltage of the field emission cathode and the electron emission device decreases, to concentrate the field efficiently and to thereby improve electron emission efficiency.
According to the electron emission device manufacturing method of the present invention, the electron emission section of the field emission cathode K is formed out of thin plate-like fine particles. Due to this, if an electric field is applied to the electron emission section, the electron beam emission section can be made sharper and it is possible to efficiently concentrate the field. Besides, the field emission cathode K is constituted such that an electron emission substance having a work function of 2 to 3 eV is bonded on the surfaces of the conductive, thin plate-like fine particles. Thus, the threshold voltage of the electron emission device is decreased, to thereby make it possible to further concentrate the electric field efficiently and to improve electron emission efficiency.